Electric trolleybuses are presently powered by an electromechanical system formed by a thyristor (chopper)-based electronic inverter and a direct current (DC) motor with non-regenerative composite excitation. A chopper is a DC to DC converter that converts DC power at a first voltage to DC power at a second voltage. Thus, electric trolleybuses are typically powered by a DC power system, as opposed to an alternating current (AC) power system. The chopper is used to control the power generated by the DC motor, which is proportional in some manner to the applied voltage to the DC motor.